Electroluminescent device



Feb. 19, 1963 w. A. THORNTON, JR 3,073,332

ELECTROLUMINESCENT DEVICE Filed Sept. 21, 1960 FIG. 2.

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nit rates 3,078,382 ELEQTROLUMHQESCENT DEVICE William A. Thornton, In,Cranford, Ni, assigns: to Westinghouse Eiectric Corporation, EastPittsburgh, Pa, a corporation of Pennsylvania Filed Sept. 21, 196i),Ser. No. 57,563 6 Claims. ((Il. 313-108) This invention relates toelectroluminescent devices and, more particularly, to anelectroluminescent device which displays improved performancecharacteristics.

The phenomenon of electroluminescence was first disclosed by G.Destriau, one of his earlier publications appearing in London, Edinburghand Dublin Philosophical Magazine, Series 7, volume 38, No. 285, pages700-737 (October 1947). Since this early publication, electroluminescentdevices have been marketed commercially. For best initial brightness, itis preferred to embed the finely divided electroluminescent phosphor inplastic dielectric material. During operation of such devices ablackening of the plastic has been observed. This blackening impairs theappearance of the devices when in an unenergized condition and alsoabsorbs some of the light which is generated during operation. Thedegree of blackening varies somewhat, but appears to be present in allplastic-type electroluminescent devices which have been marketedcommercially.

One application for which electroluminescence is particularly adapted isin conjunction with a digital display, such as described in U.S. PatentNo. 2,922,993, dated January 26, 1960, to Sack. The preferredconstruction for this display device is shown in FIG. 1 of theaforementioned Sack patent. The blackening as described hereinbefore isparticularly noticeable with sucha digital display device, since theplastic-phosphor portions which are never energized remain quite whitein appearance, while those plastic-phosphor portions which are energizedtend to blacken during life. This blackening is particularlyobjectionable in that when the device is unenergized, a numericalindicia is visible in the form of a blackened Figure 8.

It is the general object of this invention to avoid and overcome theforegoing and other diificulties of and objections to prior-artpractices by the provision of an electroluminescent device havingimproved performance characteristics.

it is another object to provide an electroluminescent device havingimproved maintenance of initial light output.

It is a further object to provide an electroluminescent device in whichblackening of the plastic portions adjacent the phosphor is greatlyinhibited.

The aforesaid objects of the invention, and other objects which willbecome apparent as the description proceeds, are achieved by providingan electroluminescent device which comprises spaced electrodes. Includedbetween the spaced electrodes is finely divided zinc-sulfidetypeelectroluminescent phosphor embedded in plastic dielectric material. Inaccordance with the present invention, additive ammonium halide isincluded in the plastic dielectric material in amount of from 0.01% to2% by weight of the plastic dielectric material. This additive ammoniumhalide serves to inhibit blackening of the plastic during operation ofthe device, in order to improve the appearance of the device as well asthe maintenance of initial light output.

For a better understanding of the invention, reference should be had tothe accompanying drawings wherein:

FIG. 1 is a sectional elevational view of an electroluminescent devicewhich incorporates additive ammoniurn halide in the dielectric, inaccordance with the pres ent invention;

FIG. 2 is a plan view, partly broken away, of an electroluminescentdevice incorporating additive ammonium halide in accordance with thepresent invention and wherein the device electrodes are formed as aninterlacing, raster-type grid;

FIG. 3 is a sectional elevational view of an alternative deviceconstruction generally corresponding to FIG. 1, but wherein anadditional layer of dielectric material is also included between thedevice electrodes.

While the present invention can be used in conjunction with any type ofelectroluminescent device, such as the digital display device describedin the aforementioned patent to Sack, the present invention also hasutility with respect to electroluminescent devices which are intended toserve as light sources or lamps and the invention has been soillustrated and will be so described.

With specific reference to the form of the invention illustrated in thedrawings, in FIG. 1 is shown an electroluminescent device 10 whichgenerally comprises a glass foundation 12 having carried thereon a firstelectrode 14 which is formed of a thin layer of light-transmitting,electrically conducting tin oxide. Coated over the electrode 14 is alayer 16 comprising finely divided electroluminescent phosphor embeddedin plastic dielectric and coated thereover is a second electrode 18,which is formed of vacuum-metallized aluminum or copper iodide, forexample. The phosphor-dielectric layer 16 also includes the additiveammonium halide, as explained hereinafter. An alternating electricpotential is adapted to be applied across the electrodes 14 and 18 inorder to energize the electroluminescent device to light emission.

The tin oxide electrode layer 14 can be formed of other suitablelight-transmitting, electrically conducting material, such as indium ortitanium oxides or copper iodide, for example. In this specific example,the phos phor is mixed with equal parts by weight of a lighttransmittingdielectric, such as polyvinyl-chloride and the thickness of the layer 16is approximately two mils. The thickness of the layer 16 is not criticaland can be varied considerably. The relative proportions of phosphor anddielectric can also be varied.

The electroluminescent phosphor which is incorporated in any of thedevice embodiments described herein is a zinc-sulfide-typeelectroluminescent phosphor. As a specific example for preparing such aphosphor, 1000 grams of zinc sulfide are mixed with 30 grams of sulphur,12.8 grams of copper acetate, and 4.5 grams of ammonium chloride. Thismixture is fired in a partially closed container in a nitrogenatmosphere at a temperature of about 950 C. for about minutes.Thereafter the phosphor is slightly crushed, 30 grams of sulphur areadded and it is retired in a similar manner. After final firing, thephosphor is lightly crushed to reduce it to finely divided status and itis then washed in a solution which is a good solvent for cuproussulfide, but which is ao'raaea not a good solvent for zinc sulfide. Anexample of such a washing solution is a one-normal solution of sodiumcyanide, made alkaline by the addition of a small amount of sodiumhydroxide. The dried phosphor has an average particle diameter of ten totwelve microns. The particle diameter is not critical and can be varied.To date, such zinc sulfide electroluminescent phosphors are the bestknown for electroluminescence. Other activators can be used tosupplement the copper, examples being manganese or lead. In addition,the zinc sulfide can be replaced in part by cadmium sulfide, or mercurysulfide, or mixtures thereof, as described in copending application S.N.807,730, filed April 20, 1959, and owned by the present assignee. All ofthe foregoing phosphors are generally classed as zinc-sulfide-typeelectroluminescent phosphors and devices constructed in accordance withthe present invention can incorporate any of such electroluminescentphosphors.

The dielectric in which the finely divided phosphor is embeddedcomprises any suitable light-transmitting plastic which is solid at roomtemperatures and which has a reasonably high dielectric constant.Examples of suitable plastic dielectric materials are polyvinylchloride, polyvinyl acetate, copolymers of the two foregoing plastics,polystyrene, polyethylene or polyvinyl butyral. Such plastic materialsare well known for use as dielectrics in electroluminescent devices.

The device embodiment 20, as shown in FIG. 2, is fabricated generally asdisclosed in FIG. 3 of U8. Patent No. 2,684,450, dated July 20, 1954.Briefly, this device embodiment comprises interlacing, raster-typeelectrodes 22 which are formed on an insulating foundation 24. The mixedfinely divided phosphor and plastic dielectric material 26, togetherwith the additive ammonium halide of the present invention, arepositioned between the interlacing electrodes 22.

The device embodiment 28, as shown in FIG. 3, corresponds to theembodiment ll), as shown in FIG. 1, except that an additional layer 3%of high-dielectric material, such as barium titanate or titania is alsoincluded between the lamp electrodes 14 and 13. This enables theenergizing electric potential applied across the device electrodes to beincreased. Because of the high dielectric constant of the layer 3%, anincreased electric field can be applied across the phosphor-dielectriclayer 16 to increase the brightness. All of the foregoingelectroluminescent device constructions essentially comprise spacedelectrodes which include therebetween finely divided, zinc-sulfide-typeelectroluminescent phosphor embedded in plastic dielectric, with aspecified amount of additive ammonium halide.

As a specific example for fabricating an electroluminescent device inaccordance with the present invention, a phosphor-plastic-vehicle slurryis first prepared. This slurry comprises polyvinyl-chloride plasticdissolved in xylene and methyl isobutyl ketone solvent in suchproportions that the viscosity of the resulting lacquer is from 16 to 22seconds, as measured with a No. 3 Zahn Cup. lhosphor is suspended inthis lacquer to form a slurry, with the suspended phosphor and dissolvedpolyvinyl chloride present in equal parts by weight. Also added to theslurry is from 0.01% by weight to 2% by weight of the dissolved plasticof an ammonium halide such as the iodide, bromide, chloride, fluoride,or any mixtures thereof. The preferred halide is ammonium iodide and thepreferred amount of ammonium halide addition is about 0.3% by weight ofthe plastic dissolved in the lacquer. The formed slurry is sprayed orotherwise applied to the device foundation as a substantially evenlayer. When the solvent volatilizes, there remains a layer of plastichaving embedded therein the finely divided, zinc-sulfide-typeelectroluminescent phosphor and also included in the plastic is theadded ammonium iodide in the indicated additive amount. The fabricationof the electroluminescent device is then completed in accordance t withconventional practices. The phosphor-plastic layer can also be appliedby other methods such as dip coating, although spraying is preferred.

In testing the present invention, lamps were fabricated generally asdisclosed in FIG. 1. A portion of these lamps served as controls and noadditive ammonium halide was included in the plastic dielectric. Theremainder of the lamps included 0.3% by weight of ammonium iodide in theplastic. Under identical test conditions, the control lamps blackenedconsiderably after hours of operation. The lamps which incorporated theiodide displayed substantially no blackening during this period ofoperation. With respect to the maintenance of initial light output, atthe end of this indicated period of operation, the maintenance ofinitial light output for the lamps containing the additive iodide was12% better than the controls.

Apparently the blackening does not occur on or in the phosphor crystals,but in the embedding plastic dielectric material proximate the phosphorparticles. This blackening is attributed to the liberation of free zincfrom the phosphor during operation. Apparently the free zinc reacts withthe plastic to form a dark compound. By including the indicated amountof ammonium halide in the plastic, the free Zinc, on liberation, reactswith the halide to form stable zinc halide. In order to realize anyappreciable improvement, at least 0.01% by weight of additive ammoniumhalide should be included in the plastic dielectric. When more than 2%by weight of ammonium halide is included in the plastic dielectric, someyellowing of the plastic is apparent. It should be noted that withammonium halides having a lower molecular weight than the iodide, agreater molecular concentration of halide can be tolerated in theplastic without encountering the yellowing effect.

It will be recognized that the objects of the invention have beenachieved by providing a plastic-type electroluminescent device havingimproved performance characteristics in that blackening of the plasticportions adjacent the phosphor is greatly inhibited and the maintenanceof initial light output is improved.

While a best example of the invention has been illustrated and describedin detail, it is to be particularly ungerstood that the invention is notlimited thereto or there- I claim:

1. An electroluminescent device comprising, spaced electrodes, finelydivided zincsulfide-type electroluminescent phosphor embedded in plasticdielectric material and included between said spaced electrodes, andadditive ammonium halide included in said plastic dielectric material inamount of from 0.01 to 2% by weight of said plastic.

2. An electroluminescent device comprising, spaced electrodes, finelydivided Zinc-sulfide-type electrolumi nescent phosphor embedded inplastic dielectric material and included between said spaced electrodes,and additive ammonium halide included in said plastic dielectricmaterial in amount of about 0.3% by weight of said plastic.

3. An electroluminescent device comprising, spaced electrodes, finelydivided zinc-sulfide-type electroluminescent phosphor embedded inplastic dielectric material and included between said spaced electrodes,and additive ammonium iodide included in said plastic dielectricmaterial in amount of from 0.01% to 2% by weight of said plastic.

4. An electroluminescent device comprising, spaced electrodes, finelydivided zinc-sulfide-type electroluminescent phosphor embedded inplastic dielectric material and included between said spaced electrodes,and additive ammonium bromide included in said plastic dielectricmaterial in amount of from 0.01% to 2% by weight of said plastic.

5. An electroluminescent device comprising, spaced electrodes, finelydivided Zinc-sulfide-type electrolumitive ammonium fluoride included insaid plastic dielecnescent phosphor embedded in plastic dielectricmaterial tric material in amount of from 0.01% to 2% by Weight andincluded between said spaced electrodes, and addiof said plastic. tiveammonium chloride included in said plastic dielectric material in amountof from 0.01% to 2% by weight of 5 References Cited in the file of thispatent said plastic.

6. An electroluminescent device comprising, spaced UNITED STATES PATENTSelectrodes, finely divided zinc-sulfide-type electrolumi- 2,698,915Piper Jan. 4, 1955 nescent phosphor embedded in plastic dielectricmaterial 2,859,367 Larach Nov. 4, 1958 and included between said spacedelectrodes, and addi- 10 2,887,601 Bain May 19, 1959

1. AN ELECTROLUMINESCENT DEVICE COMPRISING, SPACED ELECTRODES, FINELYDIVIDED ZINC-SULFIDE-TYPE ELLECTROLUMINESCENT PHOSPHOR EMBEDDED INPLASTIC DIELECTRIC MATERIAL